Re: [PATCH bpf v2 2/2] bpf: Avoid faultable build ID reads under mm locks

[Mykyta Yatsenko <mykyta.yatsenko5@gmail.com>]

On 4/9/26 2:06 AM, Ihor Solodrai wrote:
> Sleepable build ID parsing can block in __kernel_read() [1], so the
> stackmap sleepable path must not call it while holding mmap_lock or a
> per-VMA read lock.
> 
> The issue and the fix are conceptually similar to a recent procfs
> patch [2].
> 
> Resolve each covered VMA with a stable read-side reference, preferring
> lock_vma_under_rcu() and falling back to mmap_read_lock() only long
meganit: falling back to mmap_read_trylock()?
> enough to acquire the VMA read lock. Take a reference to the backing
> file, drop the VMA lock, and then parse the build ID through
> (sleepable) build_id_parse_file().
> 
> [1]: https://lore.kernel.org/all/20251218005818.614819-1-shakeel.butt@linux.dev/
> [2]: https://lore.kernel.org/all/20260128183232.2854138-1-andrii@kernel.org/
> 
> Fixes: 777a8560fd29 ("lib/buildid: use __kernel_read() for sleepable context")
> Assisted-by: Codex:gpt-5.4
> Suggested-by: Puranjay Mohan <puranjay@kernel.org>
> Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
> ---
>   kernel/bpf/stackmap.c | 139 ++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 139 insertions(+)
> 
> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
> index 4ef0fd06cea5..de3d89e20a1e 100644
> --- a/kernel/bpf/stackmap.c
> +++ b/kernel/bpf/stackmap.c
> @@ -9,6 +9,7 @@
>   #include <linux/perf_event.h>
>   #include <linux/btf_ids.h>
>   #include <linux/buildid.h>
> +#include <linux/mmap_lock.h>
>   #include "percpu_freelist.h"
>   #include "mmap_unlock_work.h"
>   
> @@ -158,6 +159,139 @@ static inline void stack_map_build_id_set_ip(struct bpf_stack_build_id *id)
>   	memset(id->build_id, 0, BUILD_ID_SIZE_MAX);
>   }
>   
> +enum stack_map_vma_lock_state {
> +	STACK_MAP_LOCKED_NONE = 0,
> +	STACK_MAP_LOCKED_VMA,
> +	STACK_MAP_LOCKED_MMAP,
> +};
> +
> +struct stack_map_vma_lock {
> +	enum stack_map_vma_lock_state state;
> +	struct vm_area_struct *vma;
> +	struct mm_struct *mm;
> +};
> +
> +static struct vm_area_struct *stack_map_lock_vma(struct stack_map_vma_lock *lock, unsigned long ip)
> +{
> +	struct mm_struct *mm = lock->mm;
> +	struct vm_area_struct *vma;
> +
> +	if (WARN_ON_ONCE(!mm))
> +		return NULL;
> +
> +	vma = lock_vma_under_rcu(mm, ip);
> +	if (vma)
> +		goto vma_locked;
> +
> +	if (!mmap_read_trylock(mm))
> +		return NULL;
> +
> +	vma = vma_lookup(mm, ip);
> +	if (!vma) {
> +		mmap_read_unlock(mm);
> +		return NULL;
> +	}
> +
> +#ifdef CONFIG_PER_VMA_LOCK
> +	if (!vma_start_read_locked(vma)) {
> +		mmap_read_unlock(mm);
> +		return NULL;
> +	}
> +	mmap_read_unlock(mm);
> +#else
> +	lock->state = STACK_MAP_LOCKED_MMAP;
> +	lock->vma = vma;
> +	return vma;
> +#endif
> +
> +vma_locked:
> +	lock->state = STACK_MAP_LOCKED_VMA;
> +	lock->vma = vma;
> +	return vma;
> +}
> +
> +static void stack_map_unlock_vma(struct stack_map_vma_lock *lock)
> +{
> +	struct vm_area_struct *vma = lock->vma;
> +	struct mm_struct *mm = lock->mm;
> +
> +	switch (lock->state) {
> +	case STACK_MAP_LOCKED_VMA:
> +		if (WARN_ON_ONCE(!vma))
> +			break;
> +		vma_end_read(vma);
> +		break;
> +	case STACK_MAP_LOCKED_MMAP:
> +		if (WARN_ON_ONCE(!mm))
> +			break;
> +		mmap_read_unlock(mm);
> +		break;
> +	default:
> +		break;
> +	}
> +
> +	lock->state = STACK_MAP_LOCKED_NONE;
> +	lock->vma = NULL;
> +}
> +
> +static void stack_map_get_build_id_offset_sleepable(struct bpf_stack_build_id *id_offs,
> +						    u32 trace_nr)
> +{
> +	struct mm_struct *mm = current->mm;
> +	struct stack_map_vma_lock lock = {
> +		.state = STACK_MAP_LOCKED_NONE,
> +		.vma = NULL,
> +		.mm = mm,
> +	};
> +	struct file *file, *prev_file = NULL;
> +	unsigned long vm_pgoff, vm_start;
> +	struct vm_area_struct *vma;
> +	const char *prev_build_id;
> +	u64 ip;
> +
> +	for (u32 i = 0; i < trace_nr; i++) {
> +		ip = READ_ONCE(id_offs[i].ip);

I'm not sure if I understand why READ_ONCE is necessary here.

> +		vma = stack_map_lock_vma(&lock, ip);
> +		if (!vma || !vma->vm_file) {
> +			stack_map_build_id_set_ip(&id_offs[i]);
> +			stack_map_unlock_vma(&lock);
> +			continue;
> +		}
> +
> +		file = vma->vm_file;
> +		vm_pgoff = vma->vm_pgoff;
> +		vm_start = vma->vm_start;
> +
> +		if (file == prev_file) {

What if instead of caching prev_file, we cache vm_start and vm_end, and 
if the next IP is in range, reuse previous build id. This should 
optimize this code further, avoiding locks on the vma used on previous 
iteration.

> +			memcpy(id_offs[i].build_id, prev_build_id, BUILD_ID_SIZE_MAX);
> +			stack_map_unlock_vma(&lock);
> +			goto build_id_valid;
> +		}
> +
> +		file = get_file(file);
> +		stack_map_unlock_vma(&lock);
> +
> +		/* build_id_parse_file() may block on filesystem reads */
> +		if (build_id_parse_file(file, id_offs[i].build_id, NULL)) {
> +			stack_map_build_id_set_ip(&id_offs[i]);
> +			fput(file);
> +			continue;
> +		}
> +
> +		if (prev_file)
> +			fput(prev_file);
> +		prev_file = file;
> +		prev_build_id = id_offs[i].build_id;
> +
> +build_id_valid:
> +		id_offs[i].offset = (vm_pgoff << PAGE_SHIFT) + ip - vm_start;
> +		id_offs[i].status = BPF_STACK_BUILD_ID_VALID;
> +	}
> +
> +	if (prev_file)
> +		fput(prev_file);
> +}
> +
>   /*
>    * Expects all id_offs[i].ip values to be set to correct initial IPs.
>    * They will be subsequently:
> @@ -178,6 +312,11 @@ static void stack_map_get_build_id_offset(struct bpf_stack_build_id *id_offs,
>   	const char *prev_build_id;
>   	int i;
>   
> +	if (may_fault && has_user_ctx) {
> +		stack_map_get_build_id_offset_sleepable(id_offs, trace_nr);
> +		return;
> +	}
> +
>   	/* If the irq_work is in use, fall back to report ips. Same
>   	 * fallback is used for kernel stack (!user) on a stackmap with
>   	 * build_id.